In an ‘outer rotor’ design of an electrical machine such as a generator, the rotor is free to rotate about the inner stator. The rotor housing of an outer rotor, such as used in a direct-drive generator, is generally made from a rolled and welded steel barrel. The complexity and cost of machining of such a rotor housing increase with rotor size. Custom machines must be built to be able to handle large outer rotors, and much effort must be invested in ensuring that the heavy but relatively thin-walled rotor does not become distorted during handling. Such distortion or ovalization is very difficult to avoid if the rotor housing is made as a one-piece barrel, especially since some generator assembly steps are carried out with the rotor in a ‘horizontal’ position, i.e. with its axis of rotation in a horizontal alignment. The own weight of the rotor may cause it to become distorted in that position before a structural element such as a front plate can be secured in a final assembly step.
Another problem associated with such outer rotor designs is the difficulty in accessing and replacing a defective magnet pole or pole piece. In the known designs, it may be necessary to disassemble the entire back plate or brake disc of the generator in order to be able to remove the defective magnet. Such a maintenance procedure is lengthy and costly, and the generator down time may be considerable.
Another problem that can be encountered with such rotor designs is that of an uneven air-gap. This can only be dealt with to a limited extent, for example by adjusting the position of one of several stator segments. A stator segment is a portion of the cylindrical stator, for example one-twelfth, and generally carries a plurality of windings arranged between stator teeth, and mounted to a carrier structure or bedframe. Since a stator segment generally covers several magnet poles, for example in a ratio of 1:8 or more, it may be difficult or impossible to correct a local air-gap inconsistency (relating to only one or two magnet poles) by adjusting the position of one stator segment.